WEDNESDAY, Nov. 29, 2017 (HealthDay News) -- Mental illness caused by traumatic experiences in early childhood may be passed from mothers to their daughters, new research suggests.

The study involved adults whose parents had been evacuated from Finland during World War II, when they were children.

Many of the approximately 49,000 children evacuated from Finland from 1941 to 1945 were preschoolers. They were placed with foster families in Sweden. Besides being separated from their families, the children faced stresses such as adapting to their foster family and learning a new language.

Then, when they returned to Finland, they had to readjust to Finnish society.

The researchers found that the daughters of women who had been evacuated had the same increased risk for hospitalization for mood disorders such as depression and bipolar disorder as their mothers.

The daughters of female evacuees were four times more likely to have been hospitalized with a mood disorder than were the daughters of women who had stayed with their families in Finland during the war.

The risk for hospitalization for mood disorders was not higher than normal among sons or daughters of men who had been evacuated as children.

The study did not pinpoint why daughters of female evacuees had the same high risk for mental illness as their mothers. However, the researchers noted that it could be related to how the mothers' traumatic childhood experience affected their parenting or to changes in the mothers' gene function.

"Many studies have shown that traumatic exposures during pregnancy can have negative effects on offspring," said study author Stephen Gilman. He's with the U.S. National Institute of Child Health and Human Development's Division of Intramural Population Health Research.

"Here, we found evidence that a mother's childhood traumatic exposure -- in this case separation from family members during war -- may have long-lasting health consequences for her daughters," Gilman explained in an institute news release.

Study co-author Torsten Santavirta, from Uppsala University in Sweden, said the evacuation from Finland "was intended to protect children from the many harms associated with the country's wars with the Soviet Union.

"Our observation of long-term psychiatric risk that reached into the next generation is concerning and underscores the need to weigh benefits as well as potential risks when designing policies for child protection," Santavirta added.

The researchers concluded that further research is needed to learn more about how being caught up in a war affects the mental health of parents and their children and to find new ways to help families affected by war.

The study was published in online Nov. 29 in JAMA Psychiatry.

More information

The Depression and Bipolar Support Alliance has more on mood disorders.